"Our regular passengers are not going to be denied boarding because they are wearing leggings or yoga trousers", Guerin told WaPo.
Watts lives in Denver and is the founder of Moms Demand Action, a national gun safety advocacy group.
Watts said she was waiting to board a plane to Mexico for vacation when she overheard a female gate agent deny entry to a young girl wearing gray leggings and saying "I don't make the rules, I just enforce them".
"Why aren't you allowing girls to wear leggings on flights?" she wrote.
But two other girls were turned away and not allowed to board because they didn't have any other clothing, she said.
NBC News was not immediately able to locate the passengers at the center of the controversy.
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After people posted on Twitter about what happened, the company responded by copy-pasting the same line from their policy over and over again: 'UA shall have the right to refuse passengers if they are not properly clothed via our Contract of Carriage'. The provision cited by the airline, which is part of the standard airline/passenger contract, does not define "properly clothed".
According to the contract, the airline can refuse to let a passenger board if he or she is "barefoot or not properly clothed". They wear yoga trousers all of the time when flying.
"We remind all of our employees to review pass travel attire requirements before using their travel privileges", one of the tweets read. There's no way she is an employee of United Airlines.
United Airlines' head office did not respond to requests from the media for comments on Sunday morning.
Comedian Sarah Silverman said that she was not going to fly on the airline for her upcoming tour. I don't care what kind of passengers they were. "They wear yoga trousers all of the time when flying". I think this policy is arbitrary and sexist.
The United contract does not define what it means for a passenger to be "not properly clothed", and the New York Daily News pointed out that United used a photo of a woman in yoga trousers in a promotional tweet in June 2016.